1. Field of the Invention
The invention relates to a process for the production of monofilaments from polyvinylidene fluoride wherein the latter, hereinafter referred to as PVDF, is plasticized with an extruder and formed into monofilaments in an extrustion die, followed by a cooling in a liquid bath. The associated apparatus comprises an extruder with an extrusion die, preferably constructed as a multihole head, whose nozzle is provided with at least one orifice, usually a plurality of orifices arranged in a circle, and a liquid-cooled cooling tank having a guide roll and takeoff means.
2. Discussion of the Prior Art
Prior-art apparatuses for the extrusion of plastic filaments are described, for example, on pages 438 to 444 of the book KUNSTSTOFF-EXTRUDERTECHNIK, by Dr. Ing. Schenkel, Carl Hanser Verlag, Munich 1963. An important consideration in the production of monofilaments is the achievement of a smooth surface, good extensibility and minimum diameter variations, in other words, maximum roundness of monofilaments which are to have a circular cross-section. The known installations for extrusion of monofilaments from polyethylene, polypropylene, polyamides, polystyrene and polyvinyl chloride have not permitted these requirements to be met by monofilaments made of PVDF. PVDF is a partially crystalline thermoplastic material whose degree of crystallinity depends to a large extent on its thermal pretreatment history. Rapid cooling after processing yields a largely amorphous material of high transparency and good flexibility while slow cooling, particularly over the temperature range in which the recrystallization rate is highest, which in the case of PVDF is between about 130.degree. and 140.degree. C., results in a highly crystalline material which, though less transparent, with higher density has greater tensile and flexural strength and improved compression creep strength. Since the monofilaments must have both high shape retention and good stretchability, a high degree of crystallinity is desirable.
Air cooling at 110.degree. to 120.degree. C., which has proved itself in the cooling of polystyrene monofilaments, has been found to be satisfactory for the cooling of PVDF monofilaments since the latter, because of their weight, are drawn out very thin. This has resulted in considerable diameter variations and deviations from the desired cross-sectional configuration. Moreover, these monofilaments run very unsteady, given the very long cooling path needed with air cooling. On the other hand, the use of water cooling for solidification of PVDF monofilaments is not the ideal solution, either, since with water cooling the monofilaments are cooled too rapidly and therefore traverse the optimum recrystallization range too quickly, with the result that they have an insufficient degree of crystallization and hence, inadequate shape retention. When the monofilaments then pass over the guide roll and move on, they undergo undesirable deformation of their cross-section.
A further problem in the extrusion of monofilaments is posed by the required narrow tolerances and constant cross-sections of the monofilaments. These requirements must be met not only during the cooling phase of the extruded monofilaments but already during extrusion. This calls for identical lengths of the flow paths, identical flow rates, and a uniform temperature pattern for all monofilaments. Still, the additional problem that the freshly extruded monofilaments do not have a sufficiently smooth surface remains.